users_name = list()
users_name.append("刘权林")
users_name.append("李浩")
users_name.append("贾庆志")
while True:
    a = input("请输入用户名\n")
    if a in users_name:

        print("用户名存在，请重新输入")
    else:
        users_name.append(a)
        break
print(users_name)
